1. Field of the Invention
The present invention relates to a light control element whose reflection factor or transmission factor varies in response to an external stimulus such as voltage, and a method of manufacturing the light control element.
2. Description of the Related Art
A light control element having a light control layer whose reflection factor or transmission factor varies in response to an external stimulus such as voltage can be used for light switches for light communication units, light arithmetic units or the like, light bulbs for projection type display units, or back light type or reflection type of direct viewing display units and so forth.
Since it has high utilization efficiency of light and is capable of effecting multi-color light control, particularly a light control element obtained by laminating a plurality of light control layers having different display colors can be suitably used for a reflection type display unit for displaying by using external light and particularly for a reflection type display unit capable of color displaying.
As a light control element whose reflection factor or transmission factor thus varies in response to an external stimulus such as voltage, Japanese Published Unexamined Patent Application No. Hei 4-355424 discloses a light control element obtained by pinching the volume hologram optical film between two sheets of substrates, on each of which an electrode has been formed, in which a liquid crystal is dispersed and held in a cured object matrix, and which has a layer structure in which the refractive index periodically varies within.
This volume hologram optical film generates reflected light of a specified wavelength by its "layer structure in which the refractive index periodically varies" in accordance with the well-known principle of interference filter. Also, since the refractive index of the liquid crystal varies in response to an external field such as voltage, the reflection factor can be controlled from a light transmitted state at a reflection factor of 0% to a light reflected state at a fixed reflection factor by varying the external field.
In this case, as a concrete form of the "layer structure in which the refractive index periodically varies", in the 0010th column and 0011th column of the Japanese Published Unexamined Patent Application No. Hei 4-355424, there are displayed "layer structure in which the content of liquid crystals periodically varies within the film" and "layer structure in which the oriented state of liquid crystals periodically varies within the film".
Also, in the 0017th column of the same Published Unexamined Patent Application, there is described "Since a difference .DELTA.n in the refractive index (within one period) relates to the (spectrum) half-amplitude level of diffraction efficiency and diffracted light, the greater the better, and the difference in the refractive index between these layers is set to at least 0.02, preferably 0.05 or more, or particularly preferably 0.10 or more".
Also, in the 0032nd column and 0036th column of the same Published Unexamined Patent Application, as a method of manufacturing volume hologram optical film, there is disclosed a method of periodically separating high polymer molecules by irradiating two light beams of coherence onto mixed liquid consisting of a photo-setting uncured object and a liquid crystal to cure the uncured object along an interference pattern formed by these light beams.
As pointed out also in Japanese Published Unexamined Patent Application No. Hei 5-181403, however, it has conventionally been difficult in the volume hologram optical film to increase the difference in the refractive index between adjacent layers.
In the same Published Unexamined Patent Application, as a method of increasing the difference in the refractive index between adjacent layers, there is disclosed a method, comprising steps of irradiating two light beams of coherence onto mixed liquid consisting of a photo-setting uncured object containing a material indicating the liquid crystal property capable of polymerization reaction and a liquid crystal which does not effect polymerization reaction to cure the uncured object along an interference pattern formed by these light beams, and thereafter, of curing the entire mixed liquid, in which, in either or both of those two steps, an external field such as an electric field or a magnetic field is applied to the mixed liquid.
The method disclosed as the second embodiment in the same Published Unexamined Patent Application will be described in conjunction with FIG. 5. According to this method, as shown in FIG. 5A, mixed liquid 60 consisting of a photo-setting uncured object containing a material indicating the liquid crystal property capable of polymerization reaction and a liquid crystal which does not effect polymerization reaction is first interposed between two substrates 11 and 12 with electrodes 41 and 42 respectively formed. Next, as shown in FIG. 5B, laser light 101, 102 is irradiated onto the mixed liquid 60 from both sides of the substrates 11 and 12 in a state in which voltage is applied between the electrodes 41 and 42. Then, as shown in FIG. 5C, ultraviolet light 103 is irradiated on the entire surface in a state in which no voltage is applied between the electrodes 41 and 42. In this respect, in the first embodiment of the same Published Unexamined Patent Application, there is disclosed a method of applying a magnetic field in place of the electric field.
According to this method, in a light control element thus obtained, in a portion cured when laser light 101, 102 is irradiated in a state in which an electric field is applied, that is, in the layer A shown in FIGS. 5B and C, the liquid crystal cured objects 66 in the high polymer molecule 65 are oriented in the electric field direction, while in a portion cured when ultraviolet light 103 is irradiated in a state in which no electric field is applied, that is, in the layer B shown in FIGS. 5C, the liquid crystal cured objects 66 are oriented at random, therefore, a difference in the refractive index is caused between layers A and B.
In this respect, in the 0041st column of the same Published Unexamined Patent Application, it is described that "it is also possible in two steps to change the oriented direction of the compound showing the liquid crystal property in the respective layers by applying the external field from two different directions". Problems to be solved by the Invention:
Even by the method described in the Japanese Published Unexamined Patent Application No. Hei 5-181403 described above, however, the difference in refractive index between adjacent layers of the light control element obtained does not still become sufficient.
More specifically, assuming the refractive index of the liquid crystal to ordinary light to be "no", and that to extraordinary light to be "ne", if the method described in the same Published Unexamined Patent Application is used, since the liquid crystals are oriented in the electric field direction in layer A shown in FIG. 5C, the refractive index of the layer A becomes "no" while since the liquid crystals are three-dimensionally oriented at random in layer B, the refractive index of the layer B becomes (ne+2no)/3. However, the refractive index of the high polymer molecule portion is ignored.
Accordingly, the difference in refractive index between layers A and B becomes (ne-no)/3, and is only one third the birefringence index (ne-no) of the liquid crystal. This is because the liquid crystals are three-dimensionally oriented at random in the layer B.
Therefore, in the light control element obtained by the method described in the Japanese Published Unexamined Patent Application No. Hei 5-181403, the half-amplitude level of the reflection spectrum during the reflection does not still become sufficiently high, and when used as a reflection type display unit, the lightness and chroma saturation do not still become sufficiently high.
Thus, by having layer structure in which the refractive index periodically varies, the present invention directs, in a light control element whose reflection factor or transmission factor varies in response to an external stimulus such as voltage, and a method of manufacturing it, to make the difference in refractive index between adjacent layers in a reflected state sufficiently great and to make the half-amplitude level of the reflection spectrum sufficiently high.